Control circuit for transmission systems



Ot.` 21, 1930. D. F. wHlTlNG 1,778,779

CONTROL CIRCUIT FOR TRANSMISSION SYSTEMS Original Filed Oct. 2l. 192'?fie A Trams/EY -mission circuits and Patented oct. 21, 1930 UNITEDSTATES DONALD F. WRITING, OF PORT PAT .PHONE LABORATORIES, INCORPORATED,F NEW YORK, N. Y., A CORPORATION OF NEW YORK CONTROL CIRCUIT FORTRANSMISSION SYSTEMS Application fled October 21, 1927, Serial No.227,693. Renewed September 5, 1929.y

This invention relates to electrical trans,-

particularly to circuits interconnecting electrical elements, such as`sion between the pick-ulg transmitters and amplifying apparatus.

In apparatus where pick-up devices, such as microphones are employed tochange acoustic vibrations into electrical vibrations, the electricalvibrations may be amplified by amplifiers. Intermediate the'microphonesand amplifiers are usually placed potenti,- ometers or the like forcontrolling the volume output of the microphones either individually orcollectively. For high quality transmisdevices and the amplifiers or towhatever apparatus the devices are connected, certainwell-knownimpedance relations should exist. For example,

4the impedances should be matched and remain substantially so during anyoperating condition of the circuit. This allows a good energy transferand prevents reflections between the elements which cause distortion.

A potentiometer for. controlling the transmission between two circuitelements while maintaining substantially constant impedance relationstherebetween during operation is disclosed in my Patent No. 1,530,633 ofMarch 24, 1925. Y

An object of this invention is to maintain substantially constantimpedance relations etween a plurality of elements, such as microphones,and another element, such as an amplifier, while varying thetransmission of any one or all of the microphones.

Another object-is to provide a mixing circuit which enables the volumeoutput of any individual microphone to be varied without interferingwith the transmission of the others.

A further object of this invention is to improve the mechanicaloperation of the potentiometers so-as to vary the gain without cuitembodying the invention and adapted to a plurality of condensertransmitters; and

ig. 2 represents diagrammatically a circuit embodying the invention andadapted to a plurality of carbon transmitters. In Fig. 1 potentiometers8, 9 and 10 control the volume output from 'amplifiers 12, 13 and 14.The inputs of these amplifiers are connected to condenser microphones16, 17 and 18. The condenser microphone-amplifier arrangement may be ofany well known type such as where the amplifier is located in closeproximity tothe microphones to reduce losses and conductor capacitances.Resistances 20, 21 and 22 are employed to substitute forthe outputimpedances of the amplifiers whenre- WASHINGTON, NEW YORK, .ASSIGNOR TOBELL TELIEI spective microphones'are disconnected from' the system.Output terminals 23 and 24 may lead to apparatus such as an amplifier ora This circuit is adapted for carbonv transmit- 4ters 41, 42 and 43 ofthe double button type which are supplied .with operating current from abattery 44.l Currents generated in the microphones aretransmitted to thepotentiometer circuit through transformers 47, 48 and 49, thetransformers serving to obtain the correct impedance relations betweenthe microphones and the output circuit. 1 Switches 50, in F ig. 1, withthe addition of a contact for' vThis tends to prevent yundue clicks andthumps in the output circuit, the output circuit being considered in and34-35, respec- 51 and 52 are thesame as those making and breaking theenergy supply circuit. These switches are constructed to have a timedelay between the making of this latter contact and the operation of theportion of the switches shown in Fig. l and vice versa, in order toallow the current to build up in the microphone circuits before theoutput circuit is connected thereto. This prevents noise of switchingoperations and transient current changes from interfering withtransmission. Condensers 55, 56 and 57 across the switch contacts reducepacking of the granular particles during switching operations. Variableresistances 58, 59 and 60 control the amount of direct current taken byeach microphone while choke coils 61, 62 and 63 prevent the alternatingcurrents of one microphone circuit from interfering with those of theother circuits. These elements do not form part of the invention but aredescribed to explain the operation of the circuits.

The output of the circuit shown in Fig. 2 is through transformer 65 andinto the input of a vacuum tube amplifier 66 shown in part. It is to beunderstood, however, that other types of circuits may be substituted forthe amplifier 66. Resistance elements 20, 21 and 22 of Fig. 2 aresubstituted for the secondary windings of transformers 47, 48 and 49,respectively, when microphones 41, 42 and 43 are disconnected.

The operation of the invention as embodied in the two circuits of Figs.1 and 2 will now be described. The slider 36 of potentiometer S movesalong a resistance element 31 in the manner of the usual potentiometerand varies the potential in the output circuit. With only thisresistance element 31, the impedance as well as the voltage impressed onthe output circuit may vary from zero to a'maximum. However, slider 36moves along resistance element 30 simultaneously with its action onelement 31,-but in a series relation which substantially maintains theimpedances in the output circuit constant. This action is duplicated inpotentiometers 9 and 10, the outputs of all the potentiometers beingconnected in series. This particular potentiometer constructioneliminates iiexible leads which might change the impedance undesirably.

To explain the action of these potentiometers an actual example will betaken. Referring to Fig. 2 the impedance of the secondary windings ofthe transformers 47, 48 and 49 will be assumed to be 50 ohms, which willalsof be the value of the resistance'ele- Ifnents 20, 21 and 22 to besubstituted thereor. has an input impedance of 200 ohms, then it isdesirable to construct the potentiometer circuit to have this value ofimpedance or substantially so. To accomplish this, the elements 31, 33and 35 will be constructed with a resistance of 190 ohms each and the Ifweassume that the transformer 65' elements 30, 32 and 34 with aresistance of 95 ohms each, the taps 7l, 72 and 73 being located so thatthe resistance of the left hand portion is 35 ohms and the remainder, ofcourse, ohms. The relative positions of the pairs of elements 30-31,32-33 and 34-35 is such that when the sliders 36, 37 and 38 are at taps71, 72 and 73, respectively, the resistance of elements 31, 33 and 35 onthe left hand side of the sliders is 70 ohms and the resistance on theright hand side 120 ohms.

With these values of the elements in this arrangement the maximum rangeof variation for the three potentiometers is 45 ohms, that is, theimpedance varies from 180 ohms to 225 ohms, while the individualpotentiometer variation is between 6() and 75 ohms. This variationoccurs when all three potentiometers are varied simultaneously from zeroto 190 ohms. In the case of the operation of a single potentiometer, theothers remaining either at zero or at the 190-ohm position, the totalvariation is only 15 ohms, which in either case is maintaining theimpedance within satisfactory limits for high quality transmisson. It isto be understood, however, that lif tapered or graduated resistanceelements are used the output impedance may be held constant, but thisexamle is based on an actual practical circuit in which the elementswere relatively inexpensive uniformly wound wire resistances.

To compute the impedance at one position-of the sliders say when thepotentiometers are at the 120-ohm position on elements 31, 33 and 35 theresistance of elements 30, 32 and 34 may be neglected since the slidersare at taps 7l, 72 and 73. The only impedance in series with the primarywinding of transformer is then the 120 ohms of elements 31, 33 and 35,shunted by the remaining 4ohms of these elements in series with the 50ohms impedance of the secondariem of transformers 47, 48 and 49,respectively.

This totals 60 ohms per potentiometer or 180 ohms for the totalimpedance. The impedance of the circuit with the sliders in otherpositions may be determined in a similar manner.

While in the specific circuits illustrated and described thepotentiometers are serially related, the invention comprehends othertypes of connection, for example, a parallel connection. The disclosureherein of particular means for carrying out the invention, therefore,and the reference to numerical values are not to be taken as in anywiselimiting the invention, but the appended claims are intended to coversuch equivalent combinations as the state of the art will allow.

What is claimed is: b1. In a transmission circuit, a plurality ofmicrophone circuits, an amplifier, and means intermediate saidmicrophone circuits lli and said amplifier for connecting saidmicrophone circuits in series with each other and for varying the outputof each microphone individually, said means matching the total impedanceof said microphone circuits and the impedance of said amplifier, andpermitting the volume output of the individual microphone circuits to bevaried while mainlis taining the total input impedance to said am'-pliier substantially matched to that of said amplifier.

2. A transmission circuit according` to claim l in which said meanscomprises potentiometers individual to said microphone circuitsconnected in series with the input of said amplifier.

3. A. transmission circuit according, to claim 1 in which said meanscomprises potentiometers each having series and shunt elements and avariable slider which traverses each element simultaneously for varyingthe volume outputof said microphone circuits, the' output circuit ofsaid potentiometers being connected in series through said sliders withthe input of said amplifier.

v4. A transmission circuit comprisingn plurality of transmitters fordetecting sound waves, an amplier for said Waves, a transformerconnected to each of said transmitters and a voltagev varying deviceconnected to each'of said transformers respectively having input'andoutput terminals, said input terminals being connected to the respectivesecondary windings of said transformers, and said output terminals beingconnected in series and to the inputcircuit of said amplifier, saidvoltage varying devices comprising means for varying the transmissionfrom said transmitters while maintaining impedance relations betweensaid transformers and said amplifier substantially constant.

v5. In a sound pick-up system, a plurality of transmitters for changingsound vibrations into electrical vibrations, an amplifier for saidelectrical vibrations, and means interconnectin said microphones andampliiier for varylng the output of at least one of said microphoneswhile maintaining voltage and impedance conditions substantiallyconstant within said system, said means comprising a plurality ofpot'entiometers serially connected to said amplifier, saidpotentiometers being in parallel individually with said transmitters. l

6. In a sound pick-up system, a-plurality s of microphones for changingsound vibrations into electrical -vibrations, a common output circuitfor'said microphones, a plurality of transformersconnect'ed to saidmicrophones, and volume varying devices comprising two resistanceelements, one of which is serially connected in said output circuit,said devices being individual to said transfrniei-IS and interconnectingsaid transformers and said output circuit forrelatively adjusting thevolume of respective microphones in small uniform stages` the output ofsaid devices being connected in said common output circuit in a seriesrelationship for maintaining the impedance of said plurality of devicessubstantially equal to the impedance of said output circuit duringadjustments of said devices and preventing the adjustment of one circuitfrom interfering with the transmission of any other circuit.

7. In a sound pick-up system, a plurality ofv transmitters for changingsound vibrations into electrical vibrations, a common output circuit,transformers for each of -said transmittersconnected between said outputcircuit and said transmitters` volume varying devices, eachhavinggseries and shunt elements, conneeted'between said transformy ersand said output circuit, said devices being connected in serieswithsaidoutput circuit and maintaining substantially constant impedancerelations between said transmitters and said Output circuit, andswitches connected to said transformers for coilnecting anddisconnecting any one of said transmitters from said output circuit.

8. vIn a system of transmission, a plnralitv of incoming circuits, anoutgoing circuit common thereto, and impedance adjusting devicesconnecting each incoming circuit to the others and to said commonoutgoingl circuit, each such device comprising a loss element connectedin shunt to a respective incoming circuit, an adjustable portion of eachsuch element being connected in series with the corresponding element ofthe next incoming circuit through the ymedium of a series adjustableloss element, the first incoming circuit having an adjustable portion ofits respective shunt element connected through a respective series losselement to one lterminal of the outgoing circuit the other terminal ofwhich is connected to the remaining terminal of the last incomingcircuit, and means for makin-g corresponding adjustments in the seriesand shunt loss elements of the respective incoming circuits to vary theloss in the individual incoming circuits While maintaining the impedancepresented to the common outgoing circuit substantial-ly constant. s

9. In a sound pick-up system, a plurality of transmitters for changingsound vibrations into' electrical output circuit, volume varyingdevices, each having series and shunt elements, connected between saidtransmitters and saidv outputV vclrcuit, sald devices being connected inseries with said output circuit and maintaining substantially constantimpedance relations between said transmitters and said output circuit.

10. In a sound pick-up system, a plurality of telephonie transmittersfor changing "sound vibrations into current waves, al comlll) mon outputcircuit, and means associated with said transmitters for controlling theamplitude of the current Wave 'of at least one Vof said transmitters,said last mentioned means comprising a plurality of impedance deviceseach having series and ,shunt elements electrically connectedin parallelindividually with said transmitters and the output connections to saidimpedance devices electrically connected in series With each other andwith said common output circuit.

11. In a transmission circuit, a plurality of incoming circuits, anamplifier, and means intermediate said incoming circuits and saidamplifier for connecting said incoming circuits in series with eachother and for varying the output level of certain of said incomingcircuits individually, said means matching the total impedance of saidincoming circuits and the impedance of)said amplifier, and permittingthe output level of the said certain incoming circuits to be variedwhile maintaining the total input impedance to said amplifiersubstantially matched to that of said amplifier.

12. In a system of transmission, a plurality of incoming circuits, anoutgoing circuit common thereto, and means associated with said incomingcircuits for controlling the impedance of at least one of said incomingcircuits, said last mentioned means comprising a plurality of impedancedevices each having series and shunt elements electrically connected inparallel individually with said incoming circuits and electricallyconnected in series with each other and with said output circuit.

13. In a transmission circuit, a pluralityl of incoming circuits, anoutgoing circuit common thereto, and means associated with said incomingcircuits for connecting said incom ing circuits in series with eachother and `for varying the output level of at least one of saidincoming' circuits, said means matching the total impedance of saidincoming circuits and the impedance of said outgoing circuit, and

permitting the output level of'said last mentioned incoming circuits tobe yaried while maintaining the total input impedance to said outgoingcircuit substantially matched to that of said outgoing circuit.

14:. In a transmission circuit, a plurality of incoming circuits, anoutgoing circuit common thereto, means associated with said incomingcircuits for connecting said incoming circuits in series with each otherand for varying the output' level of at least one of'- said incomingcircults, said means matching the total impedannce ofsaid incomingcircuits and the impedance of said outgoing circuit, and permitting theoutput level of said last mentioned incoming circuits to be varied whilemaintaining the total input impedance to said outgoing circuitsubstantially matched to that of said outgoing circuit, and 4other meansassociated with said incoming circuits for connecting and disconnectingcertain of said incoming circuits to and from said outgoing circuitWithout substantially changing the total input impedance of saidoutgoing circuit.

In Witness whereof I'hereunto' subscribe my name this 20th day ofOctober, A. D.,

DONALD F. WHITING.

